1. Field of the Invention
The present invention relates to a data transmission system for transmitting data between at least one transmitter and at least one receiver, while said data, previously organized in sequences of at least one data unit of a source protocol (source data unit), have been compressed to be transmitted in data fields of data units of a data link control protocol (transmission data units).
The invention also relates to a switch intended to be used in a transmission network, and comprising data compressing/decompressing means for the transmission/reception of data, while said data, previously organized in sequences of at least a data unit of a source protocol, have been compressed to be transmitted in data fields of data units of a data link control protocol.
The invention likewise relates to a transmitter comprising means for the transmission of data, while said data, previously organized in sequences of at least a data unit of a source protocol, have been compressed to be transmitted in data fields of data units of a data link control protocol.
The invention finally relates to a receiver comprising means for the reception of data originally organized in sequences of at least a data unit of a source protocol, then compressed and transmitted in data fields of data units of a data link control protocol.
The invention makes it particularly possible to optimize communications by reducing the quantity of data to be transmitted and thus the cost and time of transmission.
The invention has notably important applications in the field of public network transmissions, especially when these public networks employ a packet metering mode.
2. Discussion of the Related Art
Such a transmission system with previous data compression is described in French Patent Application No. 2687259, filed on 1992 by Ouest Standard Telematique. It consists of modifying the data structure during transmission to reduce the number of data units to be transmitted. The procedure described in that document consists of:
compressing the data contained in each source protocol data unit, PA1 assigning each of the obtained results to a data subfield, PA1 assigning to each data subfield a header subfield which contains information necessary for the restoration of the initial data structure and information relating to the nature of the source protocol data units, PA1 dividing this sequence of header subfields/data subfields into elements of a given length which are to form the data fields of the data units of the data link control protocol. PA1 an information component denoting the length of the dedicated data subfield, PA1 information streams occurring in the X25 header of the X25 data packets before compression (especially bit Q, which indicates whether a data packet or a check packet is concerned, and the bit M which indicates whether the X25 packet belongs to a packet sequence), PA1 an information component indicating whether the transmitted data are compressed or uncoded data.
According to this procedure, in the case where it is applied to a transmission network according to the X25 protocol of the CCITT, the header subfields particularly contain:
In the embodiment described, a header subfield occupies two octets in the data unit of the data link control protocol.
Albeit this procedure can in certain cases be satisfactory, it has the drawback of implying the systematic transmission for each data subfield of a relatively long header subfield.